In general, tape transport systems in which the supply and take-up reels are simultaneously driven by surface engagement with a driving capstan require the presence of tension in the tape span between the supply and take-up reels. The tension permits the formation of a tape pack wound on a flangeless hub which will retain its physical shape without side support and which will also withstand the high speeds and rapid accelerations and decelerations normally associated with this type of unit.
Generally such tape transports require that the tension be maintained at a constant level in order to assure uniform contact between the recording head and tape surface to enable the transduction of a flutter-free signal. An example of such a type transport is disclosed in pending U.S. application Ser. No. 388,929, filed Aug. 16, 1973 now Pat. No. 3,921,933, the disclosure of which is incorporated herein by reference. The constant tape tension in that tape transport is maintained almost entirely by braking the reel serving as the supply reel. The braking causes the surface velocity of the capstan and that of the unbraked take-up reel to slightly exceed the surface velocity of the supply reel. To accommodate the difference in speed between the supply and take-up reels the interconnecting span of tape in forced to stretch, thus generating the desired tape tension.
As an alternative to supply reel braking, other tape transports have relied upon the differential deformation phenomenon. The device in U.S. Pat. No. 3,370,803 is an example of such a tape transport. The tension is generated by using a capstan the peripheral portion of which has a capacity for elastic flow. By applying a greater force at the take-up reel/capstan interface than at the supply reel/capstan interface the rate of local acceleration by elastic flow of the capstan material at the former will exceed that at the latter. The effect is to impart a slightly higher velocity to the take-up reel with the result that tension is generated in the tape between the supply and the take-up reels by introducing, as with supply reel braking, a velocity difference between the take-up and supply reels.
As set forth in pending U.S. application Ser. No. 574,958 filed of even date with this application and entitled "IMPROVED TAPE TRANSPORT APPARATUS," now U.S. Pat. No. 4,018,402 the disclosure of which is incorporated herein by reference, there are distinct advantages in utilizing both supply reel braking and different biasing forces as tension producing means to generate components of the tension.
Regardless of the method utilized for tension generation--supply reel braking, force difference in biasing, or a combination of both--difficulties are encountered in achieving constant tape tension throughout the tape transport operation. The continually changing sizes of the reels, i.e. the large-to-small progression of the supply reel and the small-to-large progression of the take-up reel is a significant source of tension variation.
In a system which utilizes supply reel braking as a tension producing means, the use of a constant torque brake results in an increase in the force required to drive the supply reel at its periphery as the size of the reel decreases during the tape transporting operation. This increase results in a decrease in the driven speed of the supply reel. Since the tension level is a function of the difference in speed between the supply and take-up reels, the tension level will increase during the tape transporting operation. Hereinafter, this reel size-related velocity change will be referred to as the "brake effect." Although several varieties of decreasing torque brakes which maintain the driving force at a constant level are known and have been disclosed, for example, in U.S. Pat. No. 3,482,800 and in copending U.S. applications Ser. Nos. 388,929 now U.S. Pat. No. 3,921,933 and 574,958 now U.S. Pat. No. 4,018,402 filed Aug. 16, 1973, and of even date with this application, respectively, these generally are more complex and, consequently, more expensive than constant torque brakes.
Similarly, in a system which utilizes the differential deformation effect as a tension producing means, serious problems are encountered as the supply and take-up reel sizes change during the tape transporting operation if a constant force differential is maintained at the supply and take-up reel interfaces with the capstan. The deformation at the supply side increases as the supply reel decreases in size and the deformation at the take-up side decreases as the take-up reel increases in size. The result is a decrease in the supply/take-up reel velocity differential with a resulting decrease in the tape tension level as the supply reel is depleted and the take-up is augmented during tape transporting operation. Hereinafter this reelsize-related velocity change will be referred to as the ".DELTA. bias effect." One means of compensating for variations in the differential deformation by varying the reel/capstan biasing forces is set forth in above-referenced copending U.S. application Ser. No. 574,958 of even date now U.S. Pat. No. 4,018,402. As was the case with the varying torque brakes, however, a varying biasing force system is more complex and, consequently, more expensive than a constant biasing force system.
The present invention provides a method by which constant tension can be generated without the need for a biasing system or a braking system having variable characteristics to compensate for reel size changes.